Determining the Enthalpy Change of the decomposition of sodium hydrogen carbonate (conclusion + evaluation)
I found that ΔH1 = 23.4 kJ/mol and ΔH2 = 20.6 kJ/mol
2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g)
Reaction 1: 2NaCl(aq) + CO2(g) + H2O(l) → 2HCL(aq) + Na2CO3 (-∆H)
Reaction 2: 2HCl(aq) + 2NaHCO3(s) → 2NaCl(aq) + 2H2O(l) + 2CO2(g) (2 x ∆H)
The HCl, NaCl, H2O and CO2 cancel out to leave: 2NaHCO3(s) → Na2CO3(s) + H2O(l) + CO2(g)
Therefore: ∆Hr = 2 x ∆H2 + (-∆H1)
= 2 x 23.4 + 20.6 = 67.4 ± 2.3 (3.46%) kJ/mol = ∆Hr
∆Hr is positive which means that that the reaction is endothermic i.e. energy was taken in from the surroundings. In other words more energy was spent breaking bonds, which require energy to be taken in, than making them, which releases energy.
The literature value was found to be 90.0 kJ/mol
Therefore the experimental error is: 90.0-67.490.0=25.1 %
Since 25.1% > 3.46% (the apparatus error) there must have been other errors with the experiment itself to cause this.
The specific heat capacity and the density of HCl were taken as the respective values for water. This would create a result for Q which is not completely reliable (as the values for c and m would not be correct). We could improve this by working out the specific heat capacities 1.00 mol dm-3 HCl.
The parallax error is a fault in this experiment, the thermometer suffered from this effect. Results can be higher than expected when one looks tries to reach eye level one can go below eye level and read the values incorrectly. The variations in viewing angles would cause series of random errors. This error could be improved by the use of a digital thermometer which would negate the parallax error all together; with a digital thermometer readings would also be more precise as well as more accurate.
The stirring happened inconsistently and thus added a series of boosts of kinetic energy for the molecules rather than a constant stream of kinetic energy. This...